Field of the Invention
The present invention relates to a video processing apparatus, and more particularly relates to a video processing apparatus configured to combine and output a plurality of video signals.
Description of the Related Art
A conventional video processing apparatus temporarily stores an input video signal in a frame memory before outputting or displaying the video signal. When such a video processing apparatus reads the video signal from the frame memory irrespective of a frame frequency of the input video signal, an overtaking is generated in some cases between the input video signal and an output video signal, so that information (video signals) of different frames exists in one frame. Any large difference between the video signals of the different frames generates a horizontal line on a video image, which is called a tearing and degrades viewing.
A technology called frame lock prevents generation of such a tearing by reading out a video signal in synchronization with the frame frequency of the input video signal so as to appropriately adjust their phases.
Japanese Patent Laid-open No. 2007-271848 discloses a method of selecting, in a video output apparatus that generates a single display video from a plurality of input video signals and outputs the display video, a frame frequency of the display video from among those of the input video signals by a predetermined selection rule. Japanese Patent Laid-open No. 5-83686 discloses a video signal input/output apparatus that outputs a video signal converted to have a period different from that of an input video signal.
However, a method disclosed in Japanese Patent Laid-open No. 2007-271848 prompts a user to select a synchronization target video signal, and thus expects the user a relevant knowledge, which is a burden on the user. Japanese Patent Laid-open No. 2007-271848 discloses a method of prioritizing a video signal having a larger displayed area and setting it as a synchronization target, but this selection rule is not necessarily correlated with a generation of a tearing. Such a generation of a tearing makes a degradation of viewing unavoidable in some cases. Japanese Patent Laid-open No. 2007-271848 discloses a method of setting an input video having a largest content change between frames as the synchronization target, but this method requires the synchronization target to be selected after the change occurs, potentially showing, for example, a tearing to the user. In addition, measurement of the change requires a large processing load.
The configuration disclosed in Japanese Patent Laid-open No. 5-83686 does not support a combination of a plurality of input signals, and also cannot avoid generations of skipping and double reading of frames of the input video.
Another practically used technology employs a plurality of frame memories to prevent a tearing by appropriately controlling timings of writing to and read from each frame memory. However, such a configuration with a plurality of frame memories leads to a high cost.